System for verifying the integrity of cardiac complex templates

ABSTRACT

A method and system for verifying the integrity of normal sinus rhythm (NSR) templates and updating the NSR template after selected time intervals. At selected time intervals after establishing a NSR template, cardiac complexes are sensed and values for one or more cardiac parameters are measured. The values of the cardiac parameters are compared to predetermined value ranges for NSR cardiac complexes. When the values of the cardiac parameters fall within the predetermined value ranges, values for the differences between the values of the cardiac parameters for the cardiac complexes and the values for the cardiac parameters of the NSR cardiac complexes are calculated. When the values of the differences are greater than one or more threshold values, the NSR template is updated as a function of the sensed cardiac complexes.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a division of U.S. patent Ser. No. 12/685,281, filedJan. 11, 2012, now issued as U.S. Pat. No. 8,019,408, which is acontinuation of U.S. patent Ser. No. 11/287,631, filed on Nov. 28, 2005,now issued as U.S. Pat. No. 7,653,430, which is a divisional of U.S.patent Ser. No. 09/921,348, filed on Aug. 2, 2001, now issued as U.S.Pat. No. 6,996,434, which is a continuation of U.S. patent applicationSer. No. 09/267,306, filed on Mar. 12, 1999, now issued as U.S. Pat. No.6,312,388, the specifications of which are incorporated herein byreference.

FIELD OF THE INVENTION

This invention relates generally to medical devices, and moreparticularly to a system and method for verifying the integrity ofnormal sinus rhythm templates.

BACKGROUND

The heart is divided into four chambers, the left and right atrialchambers and the left and right ventricular chambers. As the heartbeats, the atrial chambers and the ventricular chambers go through acardiac cycle. The cardiac cycle consists of one complete sequence ofcontraction and relaxation of the chambers of the heart. The termssystole and diastole are used to describe the contraction and relaxationphases the chambers of the heart experience during a cardiac cycle. Insystole, the ventricular muscle cells contract to pump blood through thecirculatory system. During diastole, the ventricular muscle cells relax,causing blood from the atrial chamber to fill the ventricular chamber.After the period of diastolic filling, the systolic phase of a newcardiac cycle is initiated.

Through the cardiac cycle, the heart pumps blood through the circulatorysystem. Effective pumping of the heart depends upon five basicrequirements. First, the contractions of cardiac muscle must occur atregular intervals and be synchronized. Second, the valves separating thechambers of the heart must fully open as blood passes through thechambers. Third, the valves must not leak. Fourth, the contraction ofthe cardiac muscle must be forceful. Fifth, the ventricles must filladequately during diastole.

When the contractions of the heart are not occurring at regularintervals or are unsynchronized the heart is said to be arrhythmic.During an arrhythmia; the heart's ability to effectively and efficientlypump blood is compromised. Many different types of arrhythmias have beenidentified. Arrhythmias can occur in either the atrial chambers or inthe ventricular chambers of the heart.

Ventricular tachycardia is an arrhythmia that occurs in the ventricularchambers of the heart. Ventricular tachycardias are typified byventricular rates between 120-250 and are caused by disturbances(electrical or mechanical) within the ventricles of the heart. During aventricular tachycardia, the diastolic filling time is reduced and theventricular contractions are less synchronized and therefore lesseffective than normal. Ventricular tachycardias must be treated quicklyin order to prevent the tachycardia from degrading into a lifethreatening ventricular fibrillation.

Arrhythmias that occur in the atrial chambers of the heart are referredto generally as supraventricular tachycardias. Supraventriculartachycardias include atrial tachycardias, atrial flutter and atrialfibrillation. During certain supraventricular tachycardias, aberrantcardiac signals from the atria drive the ventricles at a very rapidrate. Such a situation occurs during paroxysmal atrial tachycardia. Thiscondition begins abruptly, lasts for a few minutes to a few hours, andthen, just as abruptly, disappears and the heart rate reverts back tonormal.

Cardioverter-defibrillators, such as implantablecardioverter-defibrillators (ICDs), have been shown to be effective inreducing the incidence of sudden cardiac death. Sudden cardiac death istypically caused by either ventricular tachycardia or ventricularfibrillation. Cardioverter-defibrillator systems operate by sensing andanalyzing cardiac signals and applying electrical energy to the heartwhen either a ventricular tachycardia or ventricular fibrillation isdetected.

One common way cardioverter-defibrillators detect cardiac arrhythmias isto sense and analyze the rate of ventricular contractions. When theventricular rate exceeds a programmed threshold value, thecardioverter-defibrillator applies electrical energy in one or morespecific patterns to treat either the ventricular tachycardia orventricular fibrillation.

An additional method cardioverter-defibrillators use to detect cardiacarrhythmias is to compare the morphology of sensed cardiac complexes totemplate cardiac complexes representative of specific cardiac rhythms.As each cardiac complex is sensed, it is compared to the templatecardiac complexes in an effort to identify and classify the sensedcardiac complex. Template cardiac complexes can be representative of avariety of cardiac complexes, including ventricular tachycardias andnormal sinus rhythm.

Template cardiac complexes are typically programmed into an implantablemedical device shortly before or after the device has been implantedinto the patient. Once the implantable medical device has been implantedinto the patient, however, the physiologic environment in which cardiacelectrodes are placed (i.e., the heart) begins to change. These changescan include an inflammatory response, localized fibrosis around theimplanted electrode and cardiac disease progression. These physiologicalchanges lead to a deterioration, or a change in the strength and themorphology of the signal sensed by the implanted medical device.Additionally, changes in a patient's medication regimen can also changethe sensing of cardiac signals by the implanted medical device.Therefore, cardiac complex templates developed before or soon afterimplanting the medical device can become less useful, or reliable, inthe process of assessing and classifying unknown cardiac complexes.

Therefore, a need exists for addressing the changes in sensed cardiacsignals as the physiological environment surrounding implanted cardiacelectrodes changes.

SUMMARY OF THE INVENTION

The present subject matter provides a system and method to verify sensednormal sinus rhythm (NSR) cardiac complexes and to use the NSR cardiaccomplexes to update a NSR template. The system and method can eitherfunction automatically after a selected time interval has expired, orafter commands have been delivered by a physician. As a result ofupdating, cardiac complexes being compared to the NSR template can beclassified more accurately than if the cardiac complexes were comparedto a NSR template that had not been updated.

Initially, a NSR template is created. In one embodiment, the NSRtemplate is created by an implantable medical device, such as animplantable cardioverter defibrillator, under the control of a patient'sattending physician. In creating a NSR template, cardiac complexes aresensed from a patient's heart. Values of one or more cardiac parametersare measured from each of the sensed cardiac complexes. In oneembodiment, an implantable cardioverter defibrillator is used to sensecardiac complexes and to measure the values of the cardiac parameters.Cardiac parameters can include, but are not limited to, ventricular andatrial cycle lengths, widths of ventricular depolarizations,atrioventricular conduction times, and R-wave amplitudes.

The values of the cardiac parameters measured from the cardiac complexesare then compared to predetermined ranges for the values of the cardiacparameters for normal sinus rhythm (NSR) complexes. Based on thiscomparison, the cardiac complexes can be determined to be, or not to be,NSR cardiac complexes. When the cardiac complexes are determined to beNSR complexes, a NSR templates is calculated as a function of these NSRcomplexes.

After a selected time interval, the NSR template is examined todetermine if it continues to accurately reflect the NSR cardiaccomplexes being sensed from the patient's heart. In one embodiment,values of cardiac parameters are measured from sensed cardiac complexesin a predetermined set of cardiac complexes. The values are thencompared to predetermined value ranges. In one embodiment, thepredetermined value ranges are individually established and programmedfor each of the cardiac parameters. Values for each cardiac parametermeasured are then compared to the corresponding predetermined valuerange established for that particular cardiac parameter.

When the values of the cardiac parameters are found to be within thepredetermined value ranges, values for cardiac signal parameterdifferences are then calculated. The cardiac signal parameter differencevalues are calculated by taking the difference of the values of thecardiac parameters from each of the sensed cardiac complexes and thevalues of the cardiac parameters for the NSR template complexes. Thevalues of the cardiac signal parameter differences are then compared tothreshold values. In one embodiment, the threshold values for each ofthe cardiac signal parameter differences are calculated by multiplying apredetermined deviation percentage and each value of the cardiacparameters for the NSR cardiac complexes used to calculate the NSRtemplate.

Based on the comparison, when the values for the cardiac signalparameter differences are found to be greater than the threshold values,the NSR template is updated as a function of the cardiac complexes.Alternatively, when the values for the cardiac signal parameterdifferences are found to be less than or equal to the threshold values,the NSR template is not updated.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow chart illustrating one embodiment of the presentsubject matter;

FIG. 2 is a flow chart illustrating one embodiment of the presentsubject matter;

FIG. 3 is a schematic of an implantable medical device; and

FIG. 4 is one embodiment of a block diagram of an implantable medicaldevice according to the present subject matter.

DETAILED DESCRIPTION

In the following detailed description, reference is made to theaccompanying drawings which form a part hereof and in which is shown byway of illustration specific embodiments in which the invention can bepracticed. These embodiments are described in sufficient detail toenable those skilled in the art to practice and use the invention, andit is to be understood that other embodiments may be utilized and thatelectrical, logical, and structural changes may be made withoutdeparting from the spirit and scope of the present invention. Thefollowing detailed description is, therefore, not to be taken in alimiting sense and the scope of the present invention is defined by theappended claims and their equivalents.

The embodiments illustrated herein are demonstrated in an implantablecardiac defibrillator (ICD), which may include numerous defibrillation,pacing, and pulse generating modes known in the art. However, theseembodiments are illustrative of some of the applications of the presentsystem, and are not intended in an exhaustive or exclusive sense. Theconcepts described herein can be used in a variety of applications whichwill be readily appreciated by those skilled in the art upon reading andunderstanding this description. For example, the present system issuitable for implementation in a variety of implantable and externaldevices.

The present subject matter allows for medical devices to examine and torecalculate or update a cardiac complex template. As previouslydiscussed, medical device systems can use a cardiac complex template toassess and classify sensed cardiac complexes. Based on theclassification of cardiac complexes, the medical device system cangenerate commands to cause the delivery of therapy to a patient's heart.

The physiologic environment in which cardiac electrodes are placed(i.e., the heart) changes from the moment the electrodes are implanted.Changes in the physiologic environment can include an inflammatoryresponse and localized fibrosis around the implanted electrode due tothe presence of the implanted electrode. These physiological changes,and other changes in cardiac disease, can lead to deterioration insensing by the implanted medical device. Additionally, changes in apatient's medication regimen can also change the sensing of cardiacsignals by the implanted medical device. Therefore, cardiac complextemplates developed soon after implanting the medical device can becomeless reliable in the process of assessing and classifying unknowncardiac complexes.

The present subject matter allows for cardiac complex templates to beexamined and, if certain predetermined conditions are met, to berecalculated. Recalculating cardiac complex templates can occur based ona determination that cardiac signals sensed during a cardiac state forwhich there is a cardiac complex template are no longer within apredetermined range of acceptability. In one embodiment, recalculationof the cardiac complex template is performed at specific time intervalsover the life of the implantable medical device. Alternatively, thecardiac complex template is recalculated, or updated, at the directionof an attending physician when the physician determines the sensedcardiac signals have deviated sufficiently from the cardiac complextemplate signals.

In one embodiment, the present subject matter is useful forrecalculating normal sinus rhythm (NSR) templates or models, such as theNSR template previously discussed. NSR templates are used in systems andmethods where morphological features from cardiac signals sensed duringa tachycardia event are compared to morphological features for cardiacsignals sensed during normal sinus rhythm (e.g., the NSR templates). Inone embodiment, procedures that compare values derived from NSR cardiaccomplexes are referred to as NSR template based procedures. In oneembodiment, NSR template based procedures require that the NSR templatebe calculated, or derived at, from a patient's NSR cardiac complexes.However, as discussed above, as the cardiac tissue surrounding thecardiac leads change (e.g., as the tissue in which the lead is embeddedchanges) sensing of the cardiac complexes changes. Thus, there is a needfor the NSR template to be updated after the medical device has beenimplanted.

Referring now to FIG. 1, there is shown an embodiment for creating anormal sinus rhythm (NSR) template. The procedure for setting the NSRtemplate is started at 100. In one embodiment, the procedure for settingthe NSR template is initiated by the patient's attending physician. Oncethe procedure is started at 100, values for one or more NSR parametersrepresentative of the patient's normal sinus rhythm are set at 104. Inone embodiment, the values of the NSR parameters are set by andprogrammed into the implantable medical device by the attendingphysician. In one embodiment, the values of the NSR parameters are basedon a patient's previously recorded cardiac data and/or the patient'scardiac history. Alternatively, the values for the NSR parameters are aset of typical values for the NSR parameters.

In one embodiment, the values for the NSR parameters are derived from,but not limited to, the following cardiac parameters: ventricular cyclelengths (e.g., time between consecutively sense R-waves), atrial cyclelengths (e.g., time between consecutively sense P-waves), standarddeviation of a plurality of ventricular cycle lengths, standarddeviation of a plurality of atrial cycle lengths, width of ventriculardepolarizations as manifested in the ventricular electrogram,atrioventricular conduction times, standard deviation of a plurality ofatrioventricular conduction times, and standard deviation of a pluralityof R-wave amplitudes as manifested in the ventricular electrogram. Inone embodiment, the NSR parameters for which values are derived areselected and programmed into the medical device system through the useof a medical device programmer. In addition, the values for the NSRparameters are also programmed into the medical device system throughthe use of the medical device programmer.

After the values for the NSR parameters are set, these initial valuesmust be checked against the values for the cardiac parameters measuredfrom the patient's NSR cardiac complexes. At 108, one or more cardiaccomplexes are sensed from cardiac signals sensed from the patient'sheart. In one embodiment, the cardiac signals are electrogram signalssensed through the use of the implanted medical device, such as an ICD,and include cardiac complexes representative of at least a portion ofthe cardiac cycle.

In one embodiment, the cardiac signals are viewed in real time on thedisplay screen of the external medical device programmer. As the cardiacsignal scrolls across the display screen, the user can select a seriesof cardiac complexes which he or she believes to be NSR complexes. Theselected cardiac complexes are then tested against the initial values ofthe NSR parameters. This procedure saves valuable time by allowing onlythose cardiac complexes believed to be NSR complexes to be checkedagainst the initial cardiac parameter values.

The implanted medical device measures and stores values for the one ormore cardiac parameters from each of the sensed cardiac complexes. Inone embodiment, the values for the one or more cardiac parameters aretaken, or derived, from the cardiac parameters whose values wereinitially set at 104. At 112, the values of the one or more cardiacparameters are used to determine whether the sensed cardiac complexesare normal sinus rhythm (NSR) complexes. In one embodiment, the valuesof the one or more cardiac parameters are compared to values of thecorresponding NSR parameters. In one embodiment, the values of the NSRparameters are those initially set at 104. In an alternative embodiment,the values of the NSR parameters are values of one or more cardiacparameters acquired during the process of setting the NSR template, anembodiment which will be described in greater detail below.

At 112, if the values of the cardiac signal parameters fall within apredetermined range of the values for the NSR parameters, then the NSRtemplate and the values for the NSR parameters are displayed at 116. Inone embodiment, displaying the NSR template and the values of the NSRparameters is done on the screen of the medical device programmer.Alternatively, the NSR template and the values of the NSR parameters aredisplayed on any suitably enabled display screen from which thisinformation can be viewed.

The NSR template is calculated as a function of sensed NSR cardiaccomplexes. In one embodiment, after qualified NSR complexes have beenidentified, a single NSR complex can be formed to represent multiple NSRcomplexes. This is done by taking a sample by sample median, mean, orother statistic of one through n sensed NSR complexes, where n is aninteger value representing the number of sensed NSR complexes. In oneembodiment, n is an integer value in the range of 2 to 20 NSR complexes.

As the one through n NSR complexes are sensed, they are aligned around acommon feature of the cardiac signal. In one embodiment, the commonfeature is a maximum deflection point of the cardiac signal as sensedduring the occurrence of the ventricular R-wave. Other common featuresof the cardiac complexes can also be used to align the sensed NSRcomplexes. Once the NSR complexes have been aligned, median or meanvalues of the cardiac complexes are used to calculate the NSR template.Once the NSR template has been created from the sample by sampleprocess, values for the NSR features are extracted from the NSR templateand stored for use with the present subject matter. In an alternativeembodiment, values for NSR features are measured for each of the sensedone through n NSR complexes. Median or mean values for the NSR featurevalues are calculated and the median or mean values are used tocalculate the NSR template.

Once the NSR template and the values for the NSR parameter have beendisplayed, the attending physician can review the information. At 120,the physician can either approve of or not approve of the NSR templateand the values for the NSR parameters proposed for use in the medicaldevice. If the NSR template and the values for the NSR parameters areapproved at 120, the NSR template and the values for the NSR parametersare programmed into the medical device system at 124 for subsequent usein discrimination procedures which rely on NSR templates and values ofNSR parameters to distinguish one arrhythmic event from another.

If, however, the values of the cardiac signal parameters do not fallwithin values of the NSR parameters at 112 or the user does not approveof the NSR template and/or the values of the NSR parameters at 120, thesystem proceeds to 128. At 128, the number of attempts at setting theNSR template is determined. In one embodiment, when 128 is reached avalue of one is added to the value of an attempt counter. In oneembodiment, a predetermined threshold value is programmed for theattempt counter. In one embodiment, the predetermined threshold value isan integer value programmed in a range of 1 to 10, where 5 is anappropriate number.

When the number of attempts at setting the NSR template exceed thepredetermined threshold value of the attempt counter, the range ofacceptable values of the NSR parameters are redefined at 132. In oneembodiment, redefining the values of the NSR parameters is done by theattending physician at the prompting the medical device programmer. Onceredefined values for the NSR parameters are programmed, additionalcardiac complexes are obtained at 108 and processed as previouslydescribed. Alternatively, if the number of attempts at setting the NSRtemplate has not exceeded the predetermined threshold value of theattempt counter, the values of the NSR parameters are not redefined andadditional cardiac complexes are obtained at 108 and processed aspreviously described.

Once a NSR template has been accepted and programmed into theimplantable medical device, it can be used in the medical device systemto assess and classify cardiac complexes. However, as the cardiacenvironment changes due to the presence of the cardiac lead (e.g., ascardiac tissue surrounding the implanted cardiac lead changes) so do thesensed cardiac signals. Changes in sensed cardiac signals necessitateupdating the NSR template so that recently sensed cardiac complexes areanalyzed with a NSR template that was also recently updated.

In order to determine if the NSR template needs to be updated, it mustfirst be examined and analyzed with respect to NSR cardiac complexesbeing sensed by the implantable medical device. Referring now to FIG. 2,there is shown one embodiment for examining a NSR template. In oneembodiment, the NSR template is examined at a selected time intervalafter calculating the NSR template. Alternatively, the NSR template isexamined at one or more selected time intervals (or times) afterdetermining the NSR template.

At 200, the system determines if the selected time interval has elapsed.In one embodiment, the selected time interval is a programmable value inthe range of 1 to 120 days, where 30 days is an appropriate length oftime. In an alternative embodiment, the selected time interval can beprogrammed to become progressively shorter or longer after the medicaldevice has been implanted in the patient. In one embodiment, the medicaldevice system can update the NSR template more frequently early in lifeof the implantable medical device and become less frequent after apredetermined length of time. For example, the system can use two ormore selected time intervals, where a first set of selected timeintervals are used to trigger updates sooner than a second set ofselected time intervals. In one embodiment, the first set of selectedtime intervals are used at an early stage after the implant of themedical device. The early stage after a medical device implant is whenthe physiological changes in the heart caused by the medical device'simplant can alter the cardiac signals sensed by the medical device.Because of the degree of change in the sensed signals at this earlystage of implant, the medical device may need to update the NSR templatemore often than when the medical device begins to receive more stablesignals at a later date. In one embodiment, the early stages of amedical device implant can be considered to be between 0 and 3 months.

Alternatively, the time and/or date for updating the NSR template can beselected by the physician. In this way, the physician can program anupdate regimen most appropriate for the type of medical device beingimplanted into the patient and/or for the type of condition that themedical device will be most likely encounter. Alternatively, thephysician can use the medical device programmer to send a signal to theimplanted medical device to begin updating the NSR template.

At 200, the elapsed time is checked against the selected time intervalto determine if the NSR template is to be updated. When the selectedtime interval expires, a predetermined set of cardiac complexes areanalyzed for the purpose of examining the NSR template. In oneembodiment, the cardiac signals are electrogram signals which containcardiac complexes, or portions of cardiac complexes (e.g., P-waves,QRS-complexes, R-waves, etc.) sensed through the use of a medical deviceas previously discussed. As the predetermined set of cardiac complexesare sensed at 204, values for the one or more cardiac parameters aremeasured from each of the sensed cardiac complexes. At 204, the cardiacparameters measured from the sensed cardiac complexes include thecardiac parameters measured to determine the values of the NSRparameters. In one embodiment, the predetermined set of cardiaccomplexes is a programmable number of cardiac complexes in the range of5 to 100, 5 to 50 or 5 to 20 cardiac complexes, where 10 cardiaccomplexes is an appropriate number.

The values of the one or more cardiac parameters measured at 204 arethen compared to one or more predetermined value ranges for the cardiacparameters at 208. In one embodiment, the values of the one or morecardiac parameters are compared to the corresponding one or morepredetermined value ranges (e.g., values ventricular intervals arecompared to a predetermined value range for ventricular intervals,values for atrial intervals are compared to a predetermined value rangefor atrial intervals, etc.) In one embodiment, the predetermined valueranges used in 208 are value ranges for the cardiac parametersprogrammed by the patient's attending physician. In an alternativeembodiment, the predetermined value ranges are the values of the cardiacparameters which were used to examine and subsequently update the NSRtemplate through the present subject matter shown in FIG. 2.

In one embodiment, the predetermined value ranges represent values forcardiac parameters that are characteristic, or representative, of thepatient's normal sinus rhythm. A major goal in examining and updatingthe NSR template is to ensure that the values of the cardiac parametersused to examine the NSR are from the patient's NSR cardiac complexes.The present system allows for verification that the NSR template updateis accomplished with, or based on, cardiac complexes sensed during apatient's NSR and not inadvertently based on cardiac complexes sensedduring a ventricular tachycardia, an atrial fibrillation, a sinustachycardia or any other non-normal sinus rhythm cardiac condition.

At 208, when the values for the one or more cardiac parameters measuredfrom the sensed cardiac signals are within the one or more predeterminedvalue ranges, the system proceeds to 212. At 212, the values of thecardiac parameters from the sensed cardiac signals are compared with thevalues of cardiac parameters for the NSR complexes used to calculate theNSR template that is now being examined.

In one embodiment, comparing these values is accomplished by firsttaking differences between the values of the cardiac parameters fromeach of the sensed cardiac signals and the values of the cardiacparameters for the NSR complexes to give cardiac signal parameterdifferences. The values of the cardiac signal parameter differences arethen compared to one or more threshold values. In one embodiment, thethreshold values are percentage deviations from the values of thecardiac parameter for the NSR complexes. The predetermined percentagedeviations can be programmed into the medical device, where the exactpercentage deviation for each cardiac parameter will be programmed bythe physician to a value that is most appropriate for that cardiacparameter. Additionally, percent deviations can also be augmented orreplaced by a check to see if the cardiac parameters are within apredetermined range.

After comparing the values of the cardiac parameters at 212, the valuesof the cardiac signal parameter differences are compared to thethreshold values at 216 to determine whether the values of the cardiacsignal parameter differences are less than the threshold values. Whenthe values of the cardiac signal parameter differences are greater thanthe threshold values, the values of the NSR parameters are replaced withthe values of the one or more cardiac parameters from the predeterminedset of cardiac complexes. The NSR template is then recalculated at 220as a function of the cardiac complexes of the predetermined set ofcardiac complexes. The medical device then records the updated NSRtemplate and the updated values of the NSR parameters for use at 212 ina subsequent examination of the NSR template.

Alternatively, if at 216, the values of the cardiac signal parameterdifferences are less than or equal to the threshold values, the valuesof the NSR parameters and the NSR template currently being examined areretained at 224 (i.e, not replaced). After either 220 or 224, the timerfor updating the template is reinitiated at 236. Once the timer forupdating has been reinitiated, the elapsed time is again checked againstthe selected time interval at 200 to determine if the NSR template is tobe updated.

Referring again to 208, if the values of the cardiac parameters measuredfrom the sensed cardiac signals are not within the predetermined valuerange, the system proceeds to 228. At 228, the number of times thevalues of the cardiac parameters have been tested against thepredetermined value range system is determined. In one embodiment, when228 is reached, a value of one (1) is added to an attempt counter value.In one embodiment, the attempt counter value has initial value of zeroat the start of the NSR template updating process.

After the one has been added to the value of the attempt counter, thenew value of the attempt counter is compared to a predeterminedthreshold value at 228. In one embodiment, the predetermined thresholdvalue is a programmable value of the number of attempts to be made atdetermining whether the cardiac parameter values measured from thesensed cardiac signals are within the predetermined value range at 208.In one embodiment, the predetermined threshold value is an integer valueprogrammed in a range of 1 to 10, where 5 is an appropriate number.

When the number of attempts at updating the NSR template and the valuesof the cardiac parameters exceeds the value of the attempt counter, theNSR template and the values of the cardiac parameters are retained(i.e., not changed) and the attempt to update the NSR template and thecardiac signal parameter is recorded in the medical device system at232. In one embodiment, a record of all successful and unsuccessful NSRtemplate updates is stored and can be displayed at follow-up visits fordiagnostic purposes.

After recording the attempt at updating the NSR template, the timer forupdating the template is reinitiated at 236. Once the timer for updatinghas been reinitiated, the elapsed time is again checked against theselected time interval at 200 to determine if the NSR template is to beupdated. In addition to reinitiating the update timer, the attemptnumber for 228 is reset to zero.

Alternatively, when the number of attempts at updating the NSR templateand the values of the cardiac parameters do not exceed the value of theattempt counter, additional cardiac parameters are measured from sensedcardiac complexes at 204. The cardiac parameter values determined at 204are then compared to the predetermined value ranges for the cardiacparameters as previously described.

The present subject matter is useful with any number of external orimplantable medical device systems which use NSR templates in analyzing,or assessing, a patient's cardiac condition. In one embodiment, cardiaccomplexes are sensed and analyzed using an implantable medical device,where the implantable medical device includes a transvenous lead systemto allow sensing of the cardiac action potentials. The transvenous leadsystem can include a rate-sensing electrode and at least onedefibrillation electrode positioned on the transvenous lead. Cardiacaction potentials sensed using defibrillation electrodes are typicallyreferred to as far-field signals (or morphology signals) and cardiacaction potentials sensed using rate sensing, or pacing, electrodes aretypically referred to as near-field signals (or rate signals).

In one embodiment, the rate-sensing electrode is a pacing tip electrodepositioned at the distal end of the transvenous lead system. Other typesof rate sensing electrodes are also considered appropriate to use withthe present subject matter. Examples of other types of rate sensingelectrodes include ring electrodes, both annular and semi-annular, asare known in the art. Rate sensing using the transvenous lead system canalso be accomplished either through unipolar or bipolar sensing methods,as are known.

In one embodiment, the implantable medical device system can employs ansingle body lead catheter sold under the trademark ENDOTAK (CardiacPacemaker, Inc./Guidant Corporation, St. Paul, Minn.) having a pacingtip electrode and two defibrillation coil electrodes. One example ofsuch a system is shown in FIG. 3. An implantable cardioverterdefibrillator (ICD) 300 is coupled to catheter 310, which is implantedto receive signals from heart 320. The catheter 310 also may be used fortransmission of pacing and/or defibrillation signals to the heart 320.In an alternative embodiment, a three defibrillation electrode system isemployed, wherein the housing of the implantable system is used as athird defibrillation electrode.

In one example, the ICD 300 senses cardiac signals from the heart 320.When the ICD 300 detects the occurrence of an arrhythmic event, the ICD300 analyzes the sensed arrhythmic complexes (i.e., the cardiac signals)of the arrhythmic event. In one embodiment, the ICD analyzes andcompares the sensed arrhythmic complexes with respect to the NSRtemplate to assess the origin of a cardiac arrhythmia (e.g., determininga ventricular tachycardia (VT) versus a supraventricular tachycardia(SVT). Based on the comparison, the ICD 300 is able to distinguish SVTevents from VT events and, depending upon the ICD's classification ofthe arrhythmic event, to provide appropriate therapy to treat the heart320. Some methods useful for distinguishing SVT events from VT eventsare presented in U.S. Pat. No. 6,223,078, entitled “Discrimination ofSupraventricular Tachycardia and Ventricular Tachycardia Events”, thespecification of which is hereby incorporated by reference in itsentirety.

Referring now to FIG. 4, there is shown one embodiment of an implantablecardiac defibrillator (ICD) 400, which may include numerousdefibrillation, pacing, and pulse generating modes known in the art. Anendocardial lead is physically and electrically coupled to the ICD 400.The endocardial lead can include at least one pacing electrode and atleast one defibrillation coil electrode as are known. In one embodiment,the endocardial lead is an ENDOTAK lead as previously described.

FIG. 4 discloses ICD 400 which includes input circuitry 410. In oneembodiment, input circuitry 410 includes a first amp 412 and a secondamp 414. The first amp 412 receives rate-signals or near-field signalsthrough the at least one pacing electrode. In one embodiment, therate-signals are sensed using a unipolar configuration, where thecardiac signals are sensed between the at least one pacing electrode andthe housing 416 of the ICD 400. Alternatively, bipolar sensing isaccomplished between two or more pacing electrodes on one or moreendocardial leads. The second amp 414 receives morphology-signals, orfar-field signals, from at least two defibrillation coil electrodeslocated on the endocardial lead.

An R-wave detector circuit 420 receives the rate-signals from the firstamp 412. The R-wave detector circuit 420 detects R-waves from therate-signals being received by the first amp 412 and conveys informationrelating to the cardiac rate to a microprocessor 424 by a data bus 426.A morphology analyzer circuit 430 receives morphology signals from thesecond amp 414. In one embodiment, the morphology analyzer circuit 430extracts and measures the values of the cardiac parameters from thesensed cardiac complexes. A template generating circuit 434 is coupledto the signal morphology analyzing circuit 430 by the bus 426. Thetemplate generating circuit 434 receives the values of the cardiacparameters. In one embodiment, the template generating circuit 434calculates, or updates, the NSR template as a function of the sensed NSRcardiac complexes.

A template comparison circuit 440 is coupled to the template circuit 434by bus 426. In one embodiment, the template comparison circuit 440compares the values of the cardiac parameters to the predetermined valueranges for the cardiac parameters. When the values of the cardiacparameters are within the predetermined value ranges the templatecomparison circuit 440 compares the values of the cardiac parametersfrom the sensed cardiac signals with the values for the cardiacparameters of the NSR cardiac complexes used to determine the NSRtemplate. As previously discussed, the cardiac signal parameterdifferences between the values of the cardiac parameters and the valuesof the cardiac parameters used to determine the NSR template arecompared to corresponding threshold values. In one embodiment, thetemplate comparison circuit 440 calculates the threshold values bymultiplying a predetermined deviation percentage and the values of thecardiac parameters for the NSR cardiac complexes used to calculate theNSR template.

After comparing the cardiac signal parameters, the template comparisoncircuit 440 compares the values of the cardiac signal parametersdifferences to the threshold values to determine whether the values ofthe cardiac signal parameter differences are less than the thresholdvalues. When the values of the cardiac signal parameter differences aregreater than the threshold values, the template generating circuit 434recalculated, or update, the NSR template as a function of the sensedcardiac complexes. In an additional embodiment, the template comparisoncircuit 440 determines the number of times the values of the cardiacparameters have been tested against the predetermined value rangesystem, and increases the value of the attempt counter during eachattempt to update the NSR template.

Power to operate the ICD 400 is supplied by a battery 448. Memory 450 isalso provided in the ICD 400, and is connected with the microprocessor424. In one embodiment, the values of the cardiac signal parameters usedto redefine the NSR template are recorded in a memory circuit 450 of theICD 400. Additionally, the attempts made to update the NSR template andthe cardiac signal parameter are also stored in the memory 450 of theICD 400. The ICD 400 further includes a transmitter receiver 454, whichcan be used to communicate with the microprocessor 424 through aprogrammer 460 as is known.

The embodiments provided herein are intended to demonstrate only some ofthe embodiments of the present system. Other embodiments utilizing thepresent subject matter are can be appreciated by those skilled in theart. For example, the concepts of the present subject matter areexpressly described in terms of cardiac complexes sensed for theQRS-wave of the heart, however, applications to other cardiac complexes,including P-wave complexes or a combination of QRS-wave and P-wavecomplexes, can be readily appreciated by those skilled in the artwithout departing from the present invention.

Also, a dual chamber implantable cardiac defibrillator can be used totake advantage of the present subject matter. In one embodiment, thedual chamber implantable cardiac defibrillator includes an ENDOTAKsingle body lead catheter implanted in the ventricular region of theheart and an atrial catheter implanted in a supraventricular region ofthe heart. This embodiment allows for ventricular near-field signals andventricular far-field signals, along with atrial near-field signals tobe sensed and analyzed by the implantable cardiac defibrillator.

Other cardiac defibrillator systems and catheter configurations may alsobe used without departing from the present system. In addition to ICDsystems, the present system may be utilized in external defibrillationsystems and in external cardiac monitoring systems. In addition toemploying endocardial leads, the present system can also utilize bodysurface leads.

Additionally, even though NSR templates were discussed herein, othertemplates, including templates for specific cardiac complexes andarrhythmic cardiac events, can also be recorded, analyzed, and up-datedusing the inventive concepts embodied in the present subject matter, andtherefore, the express teachings of this disclosure are not intended inan exclusive or limiting sense.

What is claimed is:
 1. A device, comprising: a morphology analyzeradapted to extract and measure sensed cardiac complex parameter values;a template generating circuit connected to the morphology analyzer andadapted to receive the sensed cardiac complex parameter values andgenerate a cardiac complex template; and a template comparison circuitconnected to the template generating circuit and adapted to comparesensed cardiac complex parameter values to reference cardiac parametervalues to determine if the sensed cardiac complex parameter values andthe reference cardiac parameter values have a difference greater than athreshold, wherein the template comparison circuit and the templategenerating circuit cooperate to update the cardiac complex template upondetermining that the difference is greater than the threshold.
 2. Thedevice of claim 1, wherein the cardiac complex template includes anarrhythmia template.
 3. The device of claim 1, wherein the cardiaccomplex template includes a normal sinus template.
 4. The device ofclaim 1, wherein the template comparison circuit and the templategenerating circuit cooperate to update the cardiac complex templateafter expiration of a programmed time interval.
 5. The device of claim1, wherein the template comparison circuit and the template generatingcircuit cooperate to update the cardiac complex template afterexpiration of a programmed time interval and to update the cardiaccomplex template after expiration of another programmed time interval.6. The device of claim 1, wherein the device is configured to retain thecardiac complex template if the template comparison circuit determinesthat the sensed cardiac complex parameter values are acceptable.
 7. Thedevice of claim 1, wherein the template comparison circuit is configuredto automatically compare sensed cardiac complex parameter values toreference cardiac parameter values after a programmed time period. 8.The device of claim 1, wherein the template comparison circuit isconfigured to compare sensed cardiac complex parameter values toreference cardiac parameter values in response to a physician-initiatedcommand.
 9. The device of claim 1, wherein the reference cardiacparameter values include physician-programmed values.
 10. The device ofclaim 1, wherein the template comparison circuit and the templategenerating circuit cooperate to create an initial cardiac complextemplate using one or more sensed cardiac complex values if the one ormore sensed cardiac complex parameter values are acceptable.
 11. Thedevice of claim 10, wherein the device is configured to redefineacceptable cardiac complex parameter values after a number of attemptshave been made to create the initial cardiac complex template.
 12. Thedevice of claim 1, wherein the reference cardiac parameter values aretypical cardiac complex parameter values.
 13. The device of claim 1,wherein the reference cardiac parameter values are specific to apatient's cardiac history.
 14. The device of claim 1, wherein the deviceis an implantable cardioverter defibrillator.
 15. A device, comprising:input circuitry configured to be electrically connected to at least oneelectrode to sense cardiac complexes; a morphology analyzer circuitconnected to the input circuitry and configured to measure sensedcardiac complex parameter values; a template generating circuitconnected to the morphology analyzer circuit and configured to receivethe sensed cardiac complex parameter values and generate a complexcardiac template; a template comparison circuit connected to thetemplate generating circuit and adapted to compare sensed cardiaccomplex parameter values to reference cardiac parameter values after atime interval has elapsed to determine if the sensed cardiac complexparameter values and the reference cardiac parameter values have adifference greater than a threshold; and a microprocessor operablyconnected to the input circuitry, the morphology analyzer circuit, thetemplate generating circuit, and the template comparison circuit,wherein the microprocessor includes a timer for determining the timeinterval, wherein the template comparison circuit and the templategenerating circuit cooperate to update the cardiac complex template upondetermining that the difference is greater than the threshold.
 16. Thedevice of claim 15, further comprising a display connected to themicroprocessor.
 17. The device of claim 15, further comprising adetector circuit operably connected to the input circuitry andconfigured to convey information relating to cardiac rate to themicroprocessor.
 18. The device of claim 15, further comprising a memorycircuit operably connected to the microprocessor and configured to storethe cardiac complex template and the reference cardiac parameter values.19. The device of claim 15, wherein the template comparison circuit andthe template generating circuit are configured to cooperate to updatethe cardiac complex template when the sensed cardiac complex parametervalues vary from a current cardiac complex template by a thresholdvalue.
 20. The device of claim 19, wherein the threshold value is apredetermined deviation percentage of parameter values for the currentcardiac complex.
 21. The device of claim 15, wherein the time intervalis programmable.
 22. The device of claim 21, wherein the device is animplantable device, and the time interval is programmable to beprogressively longer after the device has been implanted in a patient.